Transient Gas Flow Along Single or Bundled Electrical Cables Surrounded by a Permeable Medium

摘要

The analysis presented here provides a family of closed-form solutions which can be used to predict the penetration depth of cavity gas as it flows along a cable bundle surrounded by permeable cement. These solutions describe the transient behavior of a laminar or turbulent channel flow coupled with a Darcy or nonDarcy seepage flow. Cable-bundle flows were predicted to advance 80 to 120 metres within a minute or so, in reasonable agreement with field experience. Thus, the model seems to be useful in understanding the gross behavior of traditional stemming plans. Further analysis would be required to better model the hydrodynamic dispersion of radioactive gases, the confinement of the rock which surrounds the stemming column, and the overall gas-flow behavior within multilayer stemming plans which include fan-outs and plugs. The same basic model also describes the advance of cavity gas along a single cable surrounded by a low permeability stemming material such as cement or concrete. Even if the stemming sets tightly around each cable, mechanical deformation of the cable's insulation provides a narrow gap which can be gradually extended by the high pressure gas. However, the predicted depth of penetration is only about 1 metre within the first minute or so, based on preliminary estimates of property data for the gypsum concrete which has been recently proposed as a replacement for some parts of LLNL stemming. In summary, the scoping analyses presented here suggest that: (1) cable-bundle flow is a probable explanation for early radiation arrivals observed with traditional stemming plans, and (2) gypsum concrete stemming in fan-out regions should effectively retard the vertical flow of the gas, in spite of the seepage along the cables. Experimental studies are planned to better characterize the gypsum concrete and to test the latter assertion. (ERA citation 09:010430)